Gas turbines are well known in the art. They include a compressor which compresses the inlet air and transfers it to the combustor. The combustor has fuel added which, when ignited, heats the compressed air and produces output gases which are then expanded across a turbine to provide the power output. The combustor burns fossil fuels, such as natural gas, in the gaseous state. In many cases, water is added in the combustor to bring the combustor temperature down.
Because of the use of the fossil fuels as the combustor fuel, nitrogen oxide emissions (NO.sub.x) and carbon monoxide (CO) emissions are found in the exhaust gases. The gas turbines are designed to maintain a government imposed minimum level of NO.sub.x and CO by utilizing water injection in the combustor which is added at the same time as the fuel. Water cools the combustor and reduces the temperature of the output gases which reduces NO.sub.x but increases CO emissions. It is difficult to maintain constant NO.sub.x and minimum CO emissions because of variable operating conditions of the gas turbine such as inlet air temperature, relative humidity and various loads on the turbine, all of which cause the system to operate so as to change the emissions.
Commonly assigned U.S. Pat. No. 4,733,527 disclosed apparatus that overcame the disadvantages of the prior art by providing a system for estimating the desired water/fuel ratio to provide a substantially constant NO.sub.x emission as a function of corrected turbine speed, inlet air temperature, load and the amount of fuel being consumed as determined by the fuel metering valve position. There is a linear correlation between the water/fuel ratio and the gas generator speed corrected for inlet air temperature to maintain NO.sub.x emissions at a minimum constant level. However, the system disclosed in U.S. Pat. No. 4,733,527 has relatively flat curves representing a given water/fuel ratio for a given corrected gas generator speed. That is, the water/fuel ratio changes slowly with a given change in corrected gas generator speed. This means that the water/fuel ratio control system must be very sensitive to small changes in the water/fuel ratio for a given change in the rotational corrected speed of the gas generator for any given NO.sub.x level.
The present invention defines a method and apparatus for maintaining a constant level of NO.sub.x and minimal CO emissions from a gas turbine by injecting a calculated amount of steam into the combustion section of the gas turbine. Data has been generated that defines the relationships between a constant level of NO.sub.x emissions and the steam-to-fuel ratio required to maintain the NO.sub.x emissions at this level. The present invention utilizes these relationships to provide a method and apparatus for maintaining the amount of NO.sub.x at a constant level and minimizing CO emissions by adjusting the amount of steam injected to compensate for variations in gas turbine load, ambient temperature, relative humidity and fuel calorific heating value. The present invention also defines a means of determining the required steam-to-fuel ratio, determining the amount of fuel being used, and positioning the steam metering valve according to the amount of steam required. Steam injection tends to reduce the temperature of the combustion process which, in turn, helps reduce the amount of NO.sub.x, but increases the CO emissions.
The use of steam to reduce the amount of NO.sub.x emissions has been done in the prior art by scheduling the amount of steam to be injected either manually or automatically based on a relationship to one variable, but there are several variables that affect the amount of steam required. These variables are ambient air temperature, relative humidity, fuel heating value and turbine firing temperature. With all of these variables affecting the amount of NO.sub.x and CO emissions, a manual system or a one variable system is not practical for trying to maintain a constant level of such emissions.
In the present invention, the actual steam required is a direct calculation found by multiplying the steam-to-fuel ratio (SFR) by the fuel flow according to the equation W.sub.s =SFR x W.sub.f where W.sub.s equals steam required in pounds per hour and W.sub.f equals fuel flow in pounds per hour. The fuel flow in pounds per hour is determined by a fuel metering valve position where the position of the fuel metering valve (or angular valve opening) is directly proportional to fuel flow and is measured by an electromechancial device mounted directly on the fuel metering valve. To maintain NO.sub.x emissions at a constant level, and minimize CO emissions, steam is injected from a steam supply through a steam metering valve into the combustor where it mixes with the combustion air and fuel. This mixing tends to reduce the combustion temperature which, in turn, reduces the amount of oxides of nitrogen formation in the exhaust gases. In order to maintain a constant level of NO.sub.x over the operating range of the gas turbine, the amount of steam injected needs to vary based on fuel flow, inlet ambient air temperature, relative humidity, fuel heating value and turbine load which is a function of the turbine firing temperature. As stated earlier, the fuel flow is calculated by measuring the angular position or opening of the fuel metering valve. The relative humidity is measured by a relative humidity sensor. The fuel heating value is measured by a calorimeter. The ambient air temperature is measured by a temperature probe and the turbine firing temperature is also measured by temperature probes. These signals provide the necessary information to a controller which may be either an analog device or a digital device. The controller receives the information from the various sensors and calculates fuel flow (W.sub.f) and the steam-to-fuel ratio (SFR). The controller then multiplies these two values to obtain the required steam flow that will maintain NO.sub.x at a constant level.
Thus, the present invention relates to a method and system for utilizing steam injection to maintain substantially constant NO.sub.x from the exhaust gases of a conventional gas turbine while minimizing CO emissions.
It is an object of the present invention to calculate the steam-to-fuel ratio and the fuel consumption in pounds per hour and then multiply these two quantities together to obtain the desired steam injection in pounds per hour and develop an electrical signal that can be used to control the steam metering valve to generate the desired amount of steam injection.
It is still another object of the present invention to provide a calculated steam-to-fuel ratio based on fuel flow, inlet air temperature, relative humidity, fuel heating value and turbine load which is a function of the turbine firing temperature.
It is also an object of the present invention to determine the desired steam/fuel ratio, determine the amount of fuel being utilized, multiply the desired steam/fuel ratio times the amount of fuel being used to obtain the amount of steam that is to be used and then setting the steam metering valve according to the amount of steam desired.
It is yet another object of the present invention to provide an emissions control system that is designed to control NO.sub.x emission levels within specified limits for a range of operation of a gas turbine between minimum load and maximum load fuel flows during all ambient conditions including ambient temperature and relative humidity variations.
It is still another object of the present invention to provide a method and apparatus that will modify a steam-to-fuel ratio necessary to maintain constant emission levels for the given operating conditions on the basis of actual fuel flow into a gas turbine with the system generating a signal that represents the steam metering valve position required to inject the necessary steam into the turbine combustor to obtain the desired steam-to-fuel ratio.